Lulu and the Lampshades were a British group founded in 2009 by Heloise Tunstall-Behrens and Luisa Gerstein, who later renamed themselves Landshapes. Their contribution is the part everyone actually copies: they took A.P. Carter’s old folk song and built the cup-tapping percussion around it, uploading the result to YouTube in 2009. Every hand-clapping, cup-sliding version that followed — including the one in Pitch Perfect — descends from theirs, though almost nobody knows their name.